Animal remedies for sheep, cattle and the like are applied by a number of methods including topical or “pour-on” application, oral application, injection and nasal infusion. Each of these is typically dispensed from a “pistol grip” style dispensing means.
Typically such applicators have a piston or plunger which can be reciprocated within a barrel by squeezing and releasing a first handle relative to a second handle. The liquid to be dispensed is drawn into the barrel through an inlet via a one way inlet valve when the plunger is withdrawn inside the barrel, and is dispensed through a nozzle via an outlet valve when the plunger is extended towards the outlet valve. Such an applicator is described in the applicant's New Zealand patent No. 521084, the contents of which are herein incorporated by reference.
As is described above, conventional fluid applicators incorporate two one-way valves. These valves are referred to as the inlet valve and the outlet valve.
The valves are typically biased with springs, so that they open only when there is a predefined difference in the fluid pressure between the upstream side of the valve and the downstream side. Fluid cannot flow backwards through either valve, as flow in this direction will tend to push the valves more tightly closed.
When the applicator is at rest, both valves are closed. When the applicator is in use, it is intended that only one valve opens at a time. During the discharge stroke, the outlet valve is pushed open by the raised fluid pressure within the barrel. During the refill stroke, the inlet valve is pushed open by fluid entering the barrel (where there is now a partial vacuum).
A problem with conventional applicators is that they require a relatively large force to squeeze the handles together during the application stroke of the piston. This may be fatiguing for the operator, particularly when the applicator is used to treat a large group of animals.
The large force is required because the outlet valve of a conventional applicator is set to open only when there is a relatively large pressure in the barrel of the applicator. The reasons for this are as follows:
Firstly, the momentum of fluid travelling through the feed tube causes a pressure pulse (sometimes referred to as water hammer) at the completion of the refill stroke. The magnitude of this pressure pulse depends on factors such as the fluid velocity and the hardness of the feed tube. This pressure pulse can potentially force open both the inlet valve and the outlet valve simultaneously, and result in an unwanted discharge of fluid from the applicator. End-users strongly dislike this discharge of fluid, even if it is only a small volume.
Secondly, if the fluid supply container is held higher than the applicator (for example in a backpack) then the increased pressure can cause the fluid to flow continuously through the applicator, even when it is not squeezed, or it may drip continuously.
It would be useful to develop an applicator which allows for an outlet valve which opens under a lower pressure than the applicators of the prior art, but which does not allow any unwanted discharge of fluid from the applicator outlet.
When the applicator is used in the veterinary and/or animal husbandry fields it should preferably have the following characteristics:                be simple and reliable, suited to use in an agricultural environment.        be inexpensive to implement.        not interfere with the dose accuracy of the applicator.        work regardless of the height of the fluid source relative to the applicator.        work regardless of the viscosity of the fluid.        work regardless of the speed of discharge or refill.        work correctly during all stages of the applicator's operating cycle, including discharge, refill, and unexpected pauses in mid-stroke.        withstand attack by aggressive chemicals.        
Throughout the description and the claims, all reference to pressures are to gauge pressures, i.e. pressure relative to the ambient pressure. Therefore, a reference to zero pressure means ambient pressure. Reference to negative pressure means suction. Reference to a partial vacuum is any pressure below ambient pressure but greater than a total vacuum.
Reference to the “upstream” direction is towards the direction in the fluid flow path from which fluid enters the applicator. Reference to the “downstream” direction is to the direction in which the fluid normally flows.
The reference to any prior art in the specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in any country.